1. Field of the Invention
The present invention relates to an arrangement of devices suitable to downsize a synchrotron. The invention also relates to a synchrotron using such an arrangement and a particle therapy system using the synchrotron.
2. Description of the Related Art
In the aging society of recent years, as one of cancer treatments, a radiation treatment, which applies less load on a human body and enables the quality of life to be maintained at a high level after the treatment, has attracted attention. A particle therapy system that uses a charged particle beam (such as protons or carbon) accelerated by a synchrotron provides a high dose concentration to an affected part and has been expected as a promising system. The particle therapy system includes an injector, the synchrotron and an irradiation device. The injector supplies a charged particle beam to the synchrotron. The synchrotron accelerates the charged particle beam so that the speed of the charged particle beam becomes close to the speed of light. The irradiation device irradiates a patient with the charged particle beam extracted from the synchrotron on the basis of the position and shape of an affected part of the patient. There is a demand to reduce the size and cost of the particle therapy system in order to expand use of the particle therapy system.
As one of methods for extracting the beam from the synchrotron, there is an extraction method called a slow extraction method (resonance extraction method). The slow extraction method is different from a quick extraction method in which the entire charged particle beam is extracted during one circulation of the charged particle beam in the synchrotron. When the slow extraction method is used, the charged particle beam can be slowly extracted during a plurality of circulations of the charged particle beam in the synchrotron. The extracted beam is mainly used for a particle therapy or a physics experiment.
FIG. 4 illustrates a first example of a conventional synchrotron. A synchrotron 200 includes an injection deflection device 201 (SM, ESI), deflection magnets 202 (BM), focusing quadrupole magnets 203 (QF), defocusing quadrupole magnets 204 (QD), a radio frequency acceleration cavity 205 (RF-cavity), a resonance excitation multi-pole magnet 206 (SXFr, SXDr; only one resonance excitation multi-pole magnet is illustrated in FIG. 4), an extraction radio frequency device 207 (RF-KO), a first extraction deflector 208 (ESD) and a second extraction deflector 209 (SMI). The injection deflection device 201 causes a beam that is accelerated by a pre-accelerator 101 to be injected into the synchrotron 200. The deflection magnets 202 each deflect the injected beam and cause the beam to circulate in the synchrotron 200. The focusing quadrupole magnets 203 each cause the beam to stably circulate in the synchrotron 200 and focus the beam in a horizontal direction in order to prevent an increase in the size of the beam. The defocusing quadrupole magnets 204 each defocus the beam in the horizontal direction. The radio frequency acceleration cavity 205 accelerates and decelerates the beam. The resonance excitation multi-pole magnet 206 forms a separatrix for an oscillation (betatron oscillation) of the beam in order to cause the beam to be slowly extracted. The extraction radio frequency device 207 increases the amplitude of the oscillation of the beam and thereby leads the beam to the outside of the separatrix. The first extraction deflector 208 and the second extraction deflector 209 deflect the beam, change a path of the beam and cause the beam to be extracted from the synchrotron in order to introduce the accelerated and/or decelerated beam into an irradiation device.
In order to extract the beam from the synchrotron using the slow extraction method, the resonance excitation multi-pole magnet 206 excites resonance and forms the separatrix, and the extraction radio frequency device 207 increases the amplitude of the beam and thereby leads the beam to the outside of the separatrix. The amplitude of the beam that is led to the outside of the separatrix is further increased, and the beam propagates into the first extraction deflector 208. Then, the beam that propagates into the first extraction deflector 208 is deflected by the first extraction deflector 208, thereby being distanced from a beam that circulates in the synchrotron. The charged particle beam that is deflected by the first extraction deflector 208 is further deflected by the defocusing quadrupole magnet 204 (located on the downstream side of the first extraction deflector 208) toward the outer side of the synchrotron 200 in the horizontal direction. The charged particle beam that is deflected by the defocusing quadrupole magnet 204 toward the outer side of the synchrotron 200 in the horizontal direction passes through the deflection magnet 202 and is deflected by the focusing quadrupole magnet 203 (located on the downstream side of the defocusing quadrupole magnet 204) toward the inner side of the synchrotron 200 in the horizontal direction. Then, the charged particle beam is deflected by the second extraction deflector 209 toward the outer side of the synchrotron 200 in the horizontal direction and extracted from the synchrotron 200.